Coated optical fibers

ABSTRACT

The present invention provides a coated optical fiber having a primary coating and a secondary coating, wherein the primary coating has good attenuation loss resistance and is obtained by curing a composition having high cure speed.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. provisionalapplication No. 60/374,778, which was filed on Apr. 24, 2002.

FIELD OF THE INVENTION

[0002] The present invention relates to a coated optical fibercomprising a primary and secondary coating, to a radiation curableprimary coating composition, to a combination of a primary and secondarycoating, and to a ribbon comprising at least one of said coated opticalfibers.

BACKGROUND OF THE INVENTION

[0003] Because optical fibers are fragile and easily broken, the opticalfibers are usually coated with a protective coating system, for instancewith a soft “cushioning” primary coating that is in contact with thefiber and with a relatively hard secondary coating surrounding theprimary coating. In addition, the coating system may be used to reduceattenuation, i.e. the loss of optical power as light travels down afiber, as a result of microbending of the fiber. Providing such coatingsshould however not be at the expense of the cure speed of coatingcompositions, as this would limit line speeds in fiber drawing andtherewith increase overall production costs. Accordingly, one of theobjects of the present invention is to provide a coated optical fiberhaving a primary coating and a secondary coating, wherein the primarycoating has a high cure speed and provides good attenuation resistance.

[0004] U.S. applications Ser. Nos. 09/989,703; 09/717,337; and09/620,367 discuss primary coatings and microbending. All threeapplications are hereby incorporated in their entirety by reference.

SUMMARY OF THE INVENTION

[0005] The present invention provides a coated optical fiber having aprimary coating and a secondary coating, wherein the primary coatingprovides good attenuation resistance and is obtained by curing acomposition having a high cure speed.

BRIEF DESCRIPTION OF THE FIGURES

[0006]FIG. 1 shows a schematic illustration for an in-situ modulus testsample;

[0007]FIG. 2 is a photograph showing a set-up for measuring thecavitation resistance

DETAILED DESCRIPTION OF THE INVENTION

[0008] The present invention provides a coated optical fiber having aprimary coating and a secondary coating, wherein the primary coatingprovides good attenuation resistance and is obtained by curing acomposition having a high cure speed.

[0009] Preferably, the coated optical fiber has an attenuation increaseof less than 0.650 dB/km at 1550 nm, for instance less than 0.5 or lessthan 0.4 dB/km at 1550 nm.

[0010] Preferably, the primary coating is obtained by curing a primarycoating composition, wherein the composition has a cure dose to attain95% of the maximum attainable modulus of less than 0.65 J/cm², forinstance less than 0.55 J/cm², less than 0.45 J/cm², or less than 0.35J/cm².

[0011] Preferably, the primary coating has an in-situ modulus of lessthan 0.56 MPa, for instance less than 0.54 or 0.52 MPa. Preferably, theprimary coating also has a good in-situ modulus retention, in particularunder humid conditions. Accordingly, it is preferred that the ratio ofthe in-situ modulus of the primary coating after aging for 8 weeks at85° C. and at 85% relative humidity to the initial in-situ modulus aftercure is greater than 0.5, for instance greater than 0.75 or greater than0.9.

[0012] Furthermore, it is preferred that the primary coating has a glasstransition temperature below −10° C., for instance below −25° C., −35°C., or even below −45° C. The elongation to break of the primary coatingis preferably at least 75%, for instance at least 120% or at least 150%.

[0013] Generally, the Tg of the secondary coating is about 40° C. orhigher, for instance about 50° C. or higher or about 60° C. or higher.The tensile modulus of the secondary coating is preferably at least 200MPa, for instance at least 400 MPa or at least 500 MPa. The tensilemodulus will generally be below 3,000 MPa, for instance below 2,000 MPa.The secondary coating will preferably have an elongation to break of atleast 2%, for instance at least 10% or at least 20%.

[0014] In order to, for instance, reduce thermal stresses in the coatingsystem, it is preferred that the ratio of the expansion coefficient ofthe primary coating and the secondary coating is below 3.0, for instancebelow 2.0, such as about 1.7.

[0015] The primary coating generally will be obtained by curing aradiation curable coating composition based on (meth)acrylate functionaloligomers and radiation-curable monomers with photoinitiator(s) andadditives. Examples of additives include a stabiliser and a silanecoupling agent. The adhesion to the glass as measured according toadhesion test described in WO 99/15473, which is incorporated herein inits entirety by reference, generally is at least about 5 g in force bothat 50% RH and at 95% RH (Relative Humidity). Preferably, the adhesion isat least about 10 g in force, for instance at least about 20 g in force,at least about 50 g in force, or at least about 80 g in force, both at50% RH and 95% RH.

[0016] The primary coating composition of the present inventiongenerally comprise

[0017] (A) 20-98% by wt. of at least one oligomer having a molecularweight of about 1000 or higher, for instance 20-80% by wt. or 30-70% bywt.,

[0018] (B) 0-80% by wt. of one or more reactive diluents, for instance5-70% by wt., 10-60% by wt., or 15-60% by wt.,

[0019] (C) 0.1-20% by wt. of one or more photoinitiators for initiationof a radical polymerisation reaction, for instance 0.5-15% by wt., 1-10%by wt., or 2-8% by wt.,

[0020] (D) 0-5% by wt. of additives,

[0021] wherein the total amount adds up to 100 wt. %.

[0022] Preferably, the oligomer (A) is a urethane (meth)acrylateoligomer, comprising a (meth)acrylate group, urethane groups and abackbone. (Meth)acrylate includes acrylate as well as methacrylatefunctionality. The backbone is generally derived from a polyol which hasbeen reacted with a diisocyanate and hydroxy alkyl acrylate. However,urethane-free ethylenically unsaturated oligomers may also be used.

[0023] Examples of suitable polyols are polyether polyols, polyesterpolyols, polycarbonate polyols, polycaprolactone polyols, acrylicpolyols, and the like. These polyols may be used either individually orin combinations of two or more. Preferred polyols include polyetherpolyols, e.g. polypropylene glycol polyols such as Acclaim 4200 orAcclaim 4200N (commercially available from Lyondell), optionally incombination with polyester polyols (e.g. Priplast 3190, commerciallyavailable from Uniqema). There are no specific limitations to the mannerof polymerization of the structural units in these polyols. Any ofrandom polymerization, block polymerization, or graft polymerization isacceptable. Examples of suitable polyols, polyisocyanates andhydroxylgroup-containing (meth)acrylates are disclosed in WO 00/18696,which is incorporated herein by reference.

[0024] The reduced number average molecular weight derived from thehydroxyl number of these polyols is usually from about 50 to about25,000, preferably from about 500 to about 15,000, more preferably fromabout 1,000 to about 8,000, and most preferred, from about 1,500 to6,000. In one embodiment, the polyol(s) used in preparing the oligomerhave a molecular weight of at least 2,500 g/mol, for instance at least3,000 g/mol or at least 4000 g/mol.

[0025] The ratio of polyol, di- or polyisocyanate (as disclosed in WO00/18696), and hydroxyl group-containing (meth)acrylate used forpreparing the urethane (meth)acrylate is generally determined so thatabout 1.1 to about 3 equivalents of an isocyanate group included in thepolyisocyanate and about 0.1 to about 1.5 equivalents of a hydroxylgroup included in the hydroxyl group-containing (meth)acrylate are usedfor one equivalent of the hydroxyl group included in the polyol.

[0026] In the reaction of these three components, an urethanizationcatalyst such as copper naphthenate, cobalt naphthenate, zincnaphthenate, di-n-butyl tin dilaurate, triethylamine, andtriethylenediamine, 2-methyltriethyleneamine, is usually used in anamount from about 0.01 to about 1 wt % of the total amount of thereactant. The reaction is carried out at a temperature from about 10 toabout 90° C., and preferably from about 30 to about 80° C.

[0027] The number average molecular weight of the urethane(meth)acrylate used in the composition of the present invention ispreferably in the range from about 1,200 to about 20,000 g/mol, forinstance from about 2,200 to about 10,000 g/mol. If the number averagemolecular weight of the urethane (meth)acrylate is less than about 1000g/mol, the resin composition tends to vitrify at room temperature; onthe other hand, if the number average molecular weight is larger thanabout 20,000, the viscosity of the composition becomes high, makinghandling of the composition difficult.

[0028] Suitable reactive diluents (B) are polymerizable monofunctionalvinyl monomers and polymerizable polyfunctional vinyl monomers. Examplesof these reactive diluents are disclosed in WO 97/42130, which isincorporated herein in its entirety by reference.

[0029] Preferred reactive diluents include alkoxylated alkyl substitutedphenol acrylate, such as ethoxylated nonyl phenol acrylate, propoxylatednonyl phenol acrylate, vinyl monomers such as vinyl caprolactam,isodecyl acrylate, and alkoxylated bisphenol A diacrylate such asethoxylated bisphenol A diacrylate. In one embodiment, it is preferredto include one or more alkoxylated aliphatic polyacrylates, for instancean alkoxylated aliphatic diacrylate such as alkoxylated (e.g.propoxylated) neopentyl glycol diacrylate. In another embodiment, it ispreferred to include one or more diluents comprising one or morearomatic rings. Aromatic diluents may be helpful in embodiments where acomparatively high refractive index is desired.

[0030] Preferably, the photoinitiators (C) are free radicalphotoinitiators. Free-radical photoinitiators are generally divided intotwo classes according to the process by which the initiating radicalsare formed. Compounds that undergo unimolecular bond cleavage uponirradiation are termed Type I or homolytic photo initiators.

[0031] If the excited state photoinitiator interacts with a secondmolecule (a coinitiator) to generate radicals in a bimolecular reaction,the initiating system is termed a Type II photoinitiator. In general,the two main reaction pathways for Type II photoinitiators are hydrogenabstraction by the excited initiator or photoinduced electron transfer,followed by fragmentation.

[0032] Examples of suitable free-radical photoinitiators are disclosedin WO 00/18696 which is incorporated herein in its entirety byreference.

[0033] In one preferred embodiment of the present invention at least oneof the photoinitiators contains a phosphorous, sulfur or nitrogen atom.It is even more preferred that the photoinitiator package comprises atleast a combination of a photoinitiator containing a phosphorous atomand a photoinitiator containing a sulfur atom.

[0034] In another preferred embodiment of the invention, at least one ofthe compounds (C) is an oligomeric or polymeric photoinitiator.

[0035] As an additive (D), an amine compound can be added to the liquidcurable resin composition of the present invention to prevent generationof hydrogen gas, which causes transmission loss in the optical fibers.As examples of the amine which can be used here, diallylamine,diisopropylamine, diethylamine, diethylhexylamine, and the like can begiven.

[0036] In addition to the above-described components, various additivessuch as antioxidants, UV absorbers, light stabilizers, silane couplingagents (e.g. mercaptofunctional silane coupling agents), coating surfaceimprovers, heat polymerization inhibitors, leveling agents, surfactants,colorants, preservatives, plasticizers, lubricants, solvents, fillers,aging preventives, and wettability improvers can be used in the liquidcurable resin composition of the present invention, as required.

[0037] In general, optical fibers are coated first with a primarycoating and subsequently with a secondary coating. Suitable secondarycoatings are disclosed, for instance, in U.S. Pat. No. 6,080,483, whichis hereby incorporated in its entirety by reference. The coatings can beapplied as a wet-on-wet system (without first curing of the primary) oras a wet-on-dry system. The primary coating can be colored with a die,or secondary coatings can be colored with pigments or dies, or a clearsecondary can be further coated with an ink. The primary and secondarycoatings generally have a thickness of about 30 μm each. An ink coatinggenerally has a thickness of about 5 μm (3-10 μm).

[0038] The coated and preferably colored optical fibers can be used in aribbon comprising a plurality of said optical fibers, generally in aparallel arrangement. The plurality of optical fibers is further coatedwith one or more matrix materials in order to obtain a ribbon. Thepresent invention therefore further relates to a ribbon comprising aplurality of coated and preferably colored optical fibers, generally ina parallel arrangement, said coated optical fiber comprising at least aprimary coating according to the present invention and preferably asecondary coating according to the present invention.

[0039] The invention will be further elucidated by the followingexamples, which should be regarded as illustrating the invention and notas limiting the invention.

EXAMPLES Examples 1-6 and Comparative Examples A-C

[0040] Primary coating compositions were prepared according to theformulations listed in Table 1 below (amounts of ingredients listed inweight % relative to total weight of the composition). Also listed arephysical properties of the primary coating (see below for samplepreparation and test methods). TABLE 1 Primary coating compositionsIngredients Ex. A Ex. 1 Ex. B Ex. 2 Ex. 3 Ex. C Ex. 4 Ex. 5 Ex. 6Oligomer 1^(a) 68.60 66.15 — — — — 74.10 66.4 66.0 Oligomer 2^(b) — —52.66 — — 56.90 — — — Oligomer 3^(c) — — — 77.10 66.20 — — — —Ethoxylated Nonyl Phenol Acrylate 7.0 5.0 21.43 — 10.0 17.02 — 5.0 5.0Tridecyl acrylate 7.0 — — — — — — — — Isodecyl acrylate — 8.5 — 8.5 8.522.00 10.0 8.5 — Phenoxyethylacrylate — 4.0 — — — — — 4.0 4.0 Isobornylacrylate — — 10.71 — — — — — — Lauryl acrylate — — 6.0 — — — — — —Propoxylated (3) Trimethylolpropane triacrylate — 4.0 — — 5.0 — 4.0 4.04.0 Ethoxylated bisphenol diacrylate — 2.0 — — 2.0 — — 2.0 2.0 VinylCaprolactam 4.0 — 6.31 5.0 — — — — — Ethoxylated Aliphatic Acrylate(Ebecryl 111 from UCB 5.0 — — — — — — — — Chemicals) Propoxylated (2)Neopentyl Glycol Diacrylate (SR9003) 4.0 4.0 — 5.0 3.0 — 6.0 4.0 4.0Lucerine TPO (photoinitiator) 1.3 1.5 1.58 1.3 1.3 1.71 1.3 1.5 2.3Irgacure 184 (photoinitiator) 1.8 1.8 — 1.8 1.8 1.00 1.8 1.8 1.8 Irganox1035 (stabilizer) 0.3 — 0.32 0.3 — 0.34 — — — Irganox 3790 (stabilizer)— 1.4 — — 0.7 — — 1.4 1.4 Cyanox 1790 (stabilizer) — — — — — — 1.4 — —Tinuvin 123 — 0.4 — — — — 0.4 0.4 — Silane coupling agent 1.0 1.25 1.01.0 1.5 1.0 1.0 1.0 1.0 Properties Viscosity (mPas) 5656 6134 7500 85006673 8100 8761 6329 5850 Tensile Strength (MPa) 0.7 2.36 1.3 0.8 1.8 0.91.085 11.89 2.56 Elongation at break (%) 230 184 170 150 160 160 171 163173 Secant modulus (MPa) 0.7 0.98 0.9 0.9 0.86 1.5 1.14 1.06 0.98 Curedose to attain 95% of modulus (J/cm²) 0.21 0.47 0.3 ND 0.32 0.7 0.51 NDND Tg(° C.) −51 −47.4 ND ND −23.2 −45 −50.4 ND ND Measured shear modulusG_(measured (MPa)) 0.145 0.16 0.22 0.17 0.16 ND ND ND ND Primary coatingthickness (micron) 34 28 30 30 27 ND ND ND ND In-situ Modulus (MPa) 0.580.55 0.89 0.65 0.54 ND ND ND ND Microbending attenuation increase @ 1310nm (dB/km) 0.185 0.116 0.512 0.184 0.117 0.2 0.213 ND ND Microbendingattenuation increase @ 1550 nm (dB/km) 0.709 0.365 1.473 0.405 0.3750.454 0.628 ND ND Microbending attenuation increase @ 1700 nm (dB/km)2.61 1.168 3.807 0.960 1.148 1.54 1.806 ND ND

[0041] Cavitation resistance of above primary coatings was also measured(see below for test method and sample preparation). Several experimentswere conducted per primary coating composition. The absolute data valuewas substantially scattered, however, possibly due to differences in theangle in which the razor blade touched the sample coated optical fibers,resulting in inaccurate force distributions. The general trend observedwas (starting with the sample having the best cavitation resistance):Example A>Example 1>Examples B, 2, and 3.

[0042] The caviation resistance of Example C and Examples 4-6 was notdetermined.

[0043] In addition, the cavitation resistance of a commercial coatedoptical fiber was measured. Again, there was substantial scattering inthe data, but the the cavitation resistance appeared to be in the rangeof Example 1. The in-situ modulus of the primary coating of thecommercial fiber was determined to be 0.58 MPa. The commercial primarycoating is believed to be obtained by curing a composition having a curedose of 0.4 J/cm².

[0044] Test Methods

[0045] (i) Cure Dose

[0046] The cure speed of the compositions was determined as the curedose required to attain 95% of the maximum attainable modulus. This curedose was determined by Dose vs. Modulus curve analysis. Hereto, 6radiation-cured sample films of each composition were prepared, witheach sample film being obtained by applying an approximately 75 micronsthick composition layer on a plate and subsequently curing thecomposition layer. Each composition layer was cured with a differentdose: 0.2, 0.3, 0.5, 0.75, 1.0, and 2.0 J/cm² respectively. Sixspecimens were cut from the center portion of each prepared sample film.A Universal Testing Instrument, INSTRON Model 4201 equipped with asuitable personal computer and software “Series IX Materials TestingSystem” was used to measure the modulus of each specimen. The modulusmeasurements were then entered into the software package and thecalculations were automatically performed with a determination of theaverage modulus for each film sample. The dose-modulus curve was thencreated by plotting the modulus values vs. the dose and by fitting acurve through the data points. The “cure dose” of the coatingcomposition was determined to be the dose at which 95% of the ultimatesecant modulus is attained.

[0047] (ii) Tensile Strength, Elongation and Modulus Test Method

[0048] The tensile strength, elongation and secant modulus of curedsamples were tested using a universal testing instrument, Instron Model4201 equipped with a personal computer and software “Series IX MaterialsTesting System.” The load cells used were 4.4 Kg capacity. The ASTMD638M was followed, with the following modifications.

[0049] A drawdown of each material to be tested was made on glass plateand cured using a UV processor. A minimum of eight test specimens,having a width of 12.7.+−.0.005 mm and a length of 12.7 cm, were cutfrom the cured film. To minimize the effects of minor sample defects,sample specimens were cut parallel to the direction in which thedrawdown of the cured film was prepared. If the cured film was tacky tothe touch, a small amount of talc was applied to the film surface usinga cotton tipped applicator.

[0050] The test specimens were then removed from the substrate. Cautionwas exercised so that the test specimens were not stretched past theirelastic limit during the removal from the substrate. If any noticeablechange in sample length had taken place during removal from thesubstrate, the test specimen was discarded.

[0051] If the top surface of the film was talc coated to eliminatetackiness, then a small amount of talc was applied to the bottom surfaceof test specimen after removal from the substrate.

[0052] The average film thickness of the test specimens was determined.At least five measurements of film thickness were made in the area to betested (from top to bottom) and the average value used for calculations.If any of the measured values of film thickness deviates from theaverage by more than 10% relative, the test specimen was discarded. Allspecimens came from the same plate.

[0053] The crosshead speed was set to 25.4 mm/min, and the crossheadaction was set to “return at break”. The crosshead was adjusted to 50.8mm jaw separation. The air pressure for the pneumatic grips was turnedon and set to approximately 1.5 Kg/cm². After the Instron testinstrument had been allowed to warm-up for fifteen minutes, it wascalibrated and balanced following the manufacturer's operatingprocedures.

[0054] The temperature near the Instron instrument was measured and thehumidity was measured at the location of the humidity gauge. This wasdone just before beginning measurement of the first test specimen.

[0055] Specimens were only analyzed if the temperature was within therange 23±1.0° C. and the relative humidity was within 50±5%. Thetemperature was verified as being within this range for each testspecimen. The humidity value was verified only at the beginning and theend of testing a set of specimens from one plate.

[0056] Each test specimen was tested by suspending it into the spacebetween the upper pneumatic grips such that the test specimen wascentered laterally and hanging vertically. Only the upper grip waslocked. The lower end of the test specimen was pulled gently so that ithas no slack or buckling, and it was centered laterally in the spacebetween the open lower grips.

[0057] While holding the specimen in this position, the lower grip waslocked.

[0058] The sample number was entered and sample dimensions into the datasystem, following the instructions provided by the software package.

[0059] The temperature and humidity were measured after the last testspecimen from the current drawdown was tested. The calculation oftensile properties was performed automatically by the software package.

[0060] The values for tensile strength, % elongation, and secant, orsegment, modulus were checked to determine whether any one of themdeviated from the average enough to be an “outlier.” If the modulusvalue was an outlier, it was discarded. If there were less than six datavalues for the tensile strength, then the entire data set was discardedand repeated using a new plate.

[0061] (iii) Viscosity

[0062] The viscosity was measured using a Physica MC10 Viscometer. Thetest samples were examined and if an excessive amount of bubbles waspresent, steps were taken to remove most of the bubbles. Not all bubblesneed to be removed at this stage, because the act of sample loadingintroduces some bubbles.

[0063] The instrument was set up for the conventional Z3 system, whichwas used. The samples were loaded into a disposable aluminum cup byusing the syringe to measure out 17 cc. The sample in the cup wasexamined and if it contains an excessive amount of bubbles, they wereremoved by a direct means such as centrifugation, or enough time wasallowed to elapse to let the bubbles escape from the bulk of the liquid.Bubbles at the top surface of the liquid are acceptable.

[0064] The bob was gently lowered into the liquid in the measuring cup,and the cup and bob were installed in the instrument. The sampletemperature was allowed to equilibrate with the temperature of thecirculating liquid by waiting five minutes. Then, the rotational speedwas set to a desired value which will produce the desired shear rate.The desired value of the shear rate is easily determined by one ofordinary skill in the art from an expected viscosity range of thesample.

[0065] The instrument panel read out a viscosity value, and if theviscosity value varied only slightly (less than 2% relative variation)for 15 seconds, the measurement was complete. If not, it is possiblethat the temperature had not yet reached an equilibrium value, or thatthe material was changing due to shearing. If the latter case, furthertesting at different shear rates will be needed to define the sample'sviscous properties. The results reported are the average viscosityvalues of three test samples.

[0066] (iv) Glass Transition Temperature

[0067] The elastic modulus (E′), the viscous modulus (E″), and the tandelta (E″/E′), which is an indication of the material's T_(g), of theexamples were measured using a Rheometrics Solids Analyzer (RSA-11),equipped with: 1) a personal computer having MS-DOS 5.0 operating systemand having Rhios® software (Version 4.2.2 or later) loaded, and 2) aliquid nitrogen controller system for low-temperature operation.

[0068] The test samples were prepared by casting a film of the material,having a thickness in the range of 0.02 mm to 0.4 mm, on a glass plate.The sample film was cured using a UV processor. A specimen approximately35 mm (1.4 inches) long and approximately 12 mm wide was cut from adefect-free region of the cured film. For soft films, which tend to havesticky surfaces, a cotton-tipped applicator was used to coat the cutspecimen with talc powder.

[0069] The film thickness of the specimen was measured at five or morelocations along the length. The average film thickness was calculatedto±0.001 mm. The thickness cannot vary by more than 0.01 mm over thislength. Another specimen was taken if this condition was not met. Thewidth of the specimen was measured at two or more locations and theaverage value calculated to ±0.1 mm.

[0070] The geometry of the sample was entered into the instrument. Thelength field was set at a value of 23.2 mm and the measured values ofwidth and thickness of the sample specimen were entered into theappropriate fields.

[0071] Before conducting the temperature sweep, moisture was removedfrom the test samples by subjecting the test samples to a temperature of80° C. in a nitrogen atmosphere for 5 minutes. The temperature sweepused included cooling the test samples to about −60° C. or about −80° C.and increasing the temperature at about 1/minute until the temperaturereached about 60° C. to about 70° C. The test frequency used was 1.0radian/second. The DMA instrument produced a plot of the data on thecomputer screen. The temperature at which E′ is 1,000 MPa and E′ is 100MPa was calculated from this plot, as well as the tan delta peak. Thetemperature corresponding with the tan delta peak is reported as theglass transition temperature (Tg).

[0072] (v) In-situ Modulus

[0073] A glass optical fiber (single mode fiber having a field diameterof 10.5 micron) was coated using a primary composition according toTable 1 and a commercial secondary composition. The thus obtained coatedfiber was then placed in a metal sample fixture, as schematically shownin FIG. 1: A small portion of the coating layer was stripped in themiddle of the fiber; the length of the bottom part of the fiber was cutto be exactly 1 cm; the bottom of the fiber was inserted into a microtube in the fixture; the micro tube consisted of two half hollowcylinders; its diameter was made to be the same as the fiber outerdiameter; the fiber was tightly gripped after the screw was tightened;the gripping force on the secondary coating surface was uniform and nosignificant deformation occurred in the coating layer. The fixture withthe fiber was then mounted on DMA (same instrument as used to determinethe glass transition temperature). The metal fixture was clamped by thebottom grip. The top grip was tightened, pressing on the top portion ofthe coated fiber to the extent that it crushed the coating layer. TheDMA was set to the shear sandwich mode to measure the shear modulus ofthe primary coating. Under the force F, the primary coating layer issheared with a displacement D while essentially no deformation occurs inthe stiff secondary coating. The shear strain S (=D/T_(p)) was set to be0.05. With this low level of strain and stress, the deformation wasproven to be in the linear viscoelastic region and no delaminationoccurred at the interface of glass and primary coating. The shearmodulus G was thus obtained (values indicated in Table 1). This shearmodulus G was then corrected for stretch of the glass during measurementby the following formula:

1/G _(corrected)32 1/G _(measured)−1/G _(glass), wherein G _(glass) wastaken to be 0.85 MPa.

[0074] G corrected was then further corrected by adjusting for the realthickness of the primary coating (the thickness assumed when obtainingG_(measured) was always 30 micron), resulting in G′_(corrected) SeeTable 1 for the real thickness of the primary coatings. Finally, thein-situ modulus E was calculated with the following formula:

E=2(1+v)G _(corrected)=3G′ _(corrected), wherein v is the primarycoating Poisson ratio=0.5.

[0075] (vi) Cavitation Resistance

[0076] A Sutherland® 2000 rub tester was equipped with a fixture,replacing the heavy weight test block that this rub tester normallyuses. See FIG. 2. The left side of the fixture was locked into a jointon the tester moving arm. The bottom side was equipped with a razorblade holder. The razor blade, with the back facing down, was verticallysitting on the Q-panel, with the razor blade back edge being in completecontact with the Q-panel surface. The distance from the center of therazor blade holder to the joint is ˜1.5 in. The moving distance of therazor blade is ˜1.5 inch in half cycle. The weight of the fixture withthe razor blade is ˜200 g. The fixture was raised and a coated opticalfiber was taped on the Q-panel perpendicular to the edge of the razorblade back and in the center position. Microscope immersion oil wasdroppped on the fiber to reduce friction. The fiber had been previouslyprepared by drawing a glass optical fiber (single mode fiber having afield diameter of 10.5 micron) and coating it with the use of a primarycomposition according to Table 1 and a commercial secondary composition.

[0077] The count number of the Sutherland® 2000 rub tester was thenpreset as 3 (3 cycles, 6 times back and force in total) and speed 3 wasselected (85 cycles/min). The razor blade was subsequently lowered overthe fiber and the test was started. At the end of the cycle 3, the razorblade was raised from the fiber by hand (the moving arm runs one moreslow cycle before it stops. To avoid the possible delamination caused bythis slow rub, the razor blade should be lifted over the fiber beforethis slow rub).

[0078] The rubbed portion (i.e. 1.5 inch) of the fiber was then examinedunder the microscope at 40× magnification. The number of cavitiesobserved was noted. The more cavities observed, the poorer thecavitation resistance.

[0079] (vii) Microbending

[0080] A glass optical fiber (single mode fiber having a field diameterof 10.5 micron) was coated using a primary composition according toTable 1 and a commercial secondary composition. The microbendingresistance of the fiber was determined by determining the attenuation ofthe coated optical fiber before and after winding the fiber around adrum (diameter 600 mm) covered with sandpaper (40 μm Alox grade by 3M™).The winding force was kept constant at 4N. The attenuation increase(difference between attenuation before and after winding) was determinedat various wavelengths (as indicated in Table 1).

What is claimed is:
 1. A coated optical fiber comprising: (i) an opticalfiber; (ii) a primary coating; and (iii) a secondary coating; wherein(a) said coated optical fiber has an attenuation increase of less than0.650 dB/km at 1550 nm; (b) said primary coating is obtained by curing aprimary coating composition, said composition having a cure dose toattain 95% of the maximum attainable modulus of less than 0.65 J/cm²;and (c) said primary coating has a Tg of less than −35° C.
 2. The coatedoptical fiber of claim 1, wherein said primary coating has an in-situmodulus is less than 0.56 MPa.
 3. The coated optical fiber of claim 1,wherein said primary coating has an in-situ modulus is less than 0.54MPa.
 4. The coated optical fiber of claim 1, wherein said primarycoating has an in-situ modulus is less than 0.52 MPa.
 5. The coatedoptical fiber of claim 1, wherein said attenuation increase is less than0.500 dB/km.
 6. The coated optical fiber of claim 1, wherein saidprimary coating composition comprises an ethylenically unsaturatedoligomer.
 7. The coated optical fiber of claim 6, wherein said oligomeris prepared by reacting the following components: (1) one or morepolyisocyanates; (2) one or more polyols; and (3) one or morehydroxyfunctional (meth)acrylates.
 8. The coated optical fiber of claim7, wherein said one or more polyols includes polypropylene glycol. 9.The coated optical fiber of claim 7, wherein said one or more polyolsconsists essentially of polypropylene glycol.
 10. The coated opticalfiber of claim 7, wherein said one or more polyols includes a polyesterpolyol.
 11. The coated optical fiber of claim 7, wherein said one ormore polyols each have a molecular weight of at least 3,000 g/mol. 12.The coated optical fiber of claim 7, wherein said one or morehydroxyfunctional (meth)acrylates includes hydroxyethyl acrylate. 13.The coated optical fiber of claim 1, wherein said primary coatingcomposition comprises one or more monomers.
 14. The coated optical fiberof claim 13, wherein said one or more monomers includes an alkoxylatedacrylate monomer.
 15. The coated optical fiber of claim 13, wherein saidone or more monomers includes an alkoxylated aliphatic polyacrylatemonomer.
 16. The coated optical fiber of claim 1, wherein said cure doseis below 0.55 J/cm².
 17. The coated optical fiber of claim 1, whereinsaid primary coating has a Tg below −45° C.
 18. The coated optical fiberof claim 1, wherein said an optical fiber is an optical glass fiber.